1. Field of the Invention
This invention relates to an image display apparatus for an X-ray television imaging system producing television images of diagnostic quality from radiation patterns from a subject, coverted to light images by an image tube.
2. Description of the Prior Art
In many medical X-ray diagnostic procedures, radiopaque material is injected into, or injested by, a patient and X-rays are then directed through the patient. The radiation which is transmitted through the patient produces a fluorographic and/or a radiographic shadow image of the opaque material. This image provides diagnostic information about the tissues which surround and shape the opaque material.
A basic type of X-ray inspection is so-called "bright fluoroscopy." With bright fluoroscopy an image intensifier tube produces an intensified fluoroscopic image which is viewed selectively or concurrently by film and television cameras. Television imaging techniques produce a substantially continuous image of the region of interest of the body. The film cameras produce both single frame ("spot") photographs, and motion picture or "cine" exposures.
Television imaging is particularly useful in applications in which the areas of differing radiopacity in the subject exhibit motion or are otherwise time varying. Television, for example, can allow the physician to watch the progress of a radiopaque material ingested by the subject through the alimentary canal, or can be used to observe progress of such material injected into blood vessels.
With a typical bright fluoroscopy system, an X-ray image tube is axially aligned with an X-ray beam. An X-ray stimulated light image is produced on an input phosphor of the image tube. This light image causes an adjacent electron emitting layer to emit electrons that are accelerated against an output phosphor to produce a relatively small and intensified output image.
A so-called "optical cube" is coupled to the image tube. The optical cube typically has an axially aligned television camera and spot and cine film cameras positioned with their optical axes normal to an extension of the image tube axis. Transmissive mirrors known as "beam splitters," are used to selectively distribute the light among the two film cameras and the television camera. The output of the television camera is transmitted to one or more monitors for dynamic viewing of a study being conducted.
Many variations of the described system have been proposed or produced. For example, film cameras have been used to produce either single frame or moving picture images of the output of a television monitor. While the output of the television monitor has been photographed, the resolution of photographic images of television monitors has been poor as compared to that of images produced by cameras that are optically coupled to the output phosphor of the image tube.
Systems have been produced in which the X-ray source was pulsed in synchronism with the field scan rate of a television camera at about 1-6 pulses per second. This reduced the X-ray dosage received by the patient in the course of an examination, but still maintained an apparently continuous image on the television monitor so long as cine and spot films were not being taken.
The system described here, while furnishing adequate television fluoroscopic images for general observations and some diagnostic purposes, has not provided television images of sufficiently high resolution for use as actual diagnostic tools in many other procedures. This loss in resolution is due to losses in optics and in production of the television image.
The ability of an optical element (including, e.g., an image tube, a lens, a television camera, or a cathode ray tube display) to resolve images is often described in terms of its "modulation transfer function" (MTF). Normally, the ability of an optical system to resolve a portion of an image decreases as the fineness of detail of the image portion (the number of lines per unit distance) increases. This degraded resolution is manifested as a reduction in the contrast between the light areas and the dark areas of the image portion. The number of lines per unit distance is frequently expressed as "line pairs per millimeter," and is known generically as the "spatial frequency" of the image portion of interest.
The modulation transfer function (MTF) is defined as the function of the percent of contrast in the image with respect to the spatial frequency of the features of the image portion. The percent of contrast is defined as a function of the maximum light energy per unit area (brightness) in the light areas of the picture portion under consideration (Bmax) and the energy per unit area of the darker areas (Bmin). Specifically, the contrast ratio is defined as: ##EQU1##
Tests on several prior art X-ray television imaging systems have shown these systems as having a total "modulation transfer function" defined approximately by the ranges of points designated as follows:
______________________________________ Line pairs/m.m. % Contrast ______________________________________ 0.25 50-65 0.50 15-45 1.00 0-5 ______________________________________
Because of the limited resolution of television imaging, examiners have employed the television images mostly for general observation to determine regions of interest.
Once the region of interest is determined, radiographs or films of the image tube output are used for the final diagnosis.
The limited resolution of prior systems resulted in part from brightness losses in the optical coupling between the output phosphor of the image tube and the television pickup tube. Attempts were made to improve prior systems with circuitry for enhancing the contrast of the television image, but such attempts failed to improve resolution, because of the deterioration of the signal to noise ratio in the television image. Other attempts to improve resolution included the use of very large aperture lenses for the lens assembly, but such lenses are quite expensive and suffer from significant aberrations which distort the transmitted image. The necessity for providing the mirror structure associated with the lens assembly (for enabling the use of the second film camera) resulted in substantial vignetting of the image. The television pickup tube employed in the previous system also suffered a deterioration in resolution when operated in the pulsed mode, such as required in X-ray imaging systems. The television monitors used were not readily engageable for operation with downstream optical components of the system, such as film cameras, and these CRT displays suffered deteriorated modulation transfer functions when image brightness was increased substantially.
The magnification of the output phosphor image (which was about 20 millimeters in diameter) was required of the lens assembly to take full advantage of the input face area of the television pickup tube, which is about 35 millimeters in diameter. The required magnification of the image tube output image aggravated the loss of brightness in the lens assembly in the course of transmission from the output phosphor to the pickup tube, since an image becomes dimmer when magnified.
The prior system failed to provide television images of sufficient resolution also because of noise in the electrical television composite signals. Both the noise and the poor resolution result from severe restrictions on the intensity of X-rays which may safely be directed through the subject. With such low X-ray intensities, the visible image at the image tube output must necessarily be relatively dim, and of low contrast, and the prior system did not have sufficient sensitivity to resolve these low contrast images. Improvement of the system has been attempted by various means of generalized contrast enhancement of the television image by amplification, but these attempts have not been very successful. When the signals from the television camera are generally amplified to increase contrast, noise present in the television signals is also amplified, and appears objectionably in the television image produced.
Several other means have been attempted to enhance the sensitivity and hence the resolution of the prior system. One of these has included the use of very large aperture lenses for the lens assembly between the output phosphor and the television camera input, to gather as much light as possible from the image tube output and transmit it to the television camera. Such lenses have had apertures of as much as f0.75. The lens assemblies have included a collimating lens positioned to receive the image from the output phosphor, and a focusing lens to focus the image on the television pickup tube input. The need for magnification of the output phosphor image dictates that the collimating and focusing lenses be unmatched.
Large aperture lenses are expensive, and the expense is further aggravated by the necessity for grinding two different lenses for each system. Moreover, the use of two lenses having different focal lengths (i.e., nonsymmetrical) causes the lens aberrations which normally occur in each of the lenses to be additive and to distort the transmitted image.
In the prior system, the two lenses of the lensing system have been separated by several millimeters, to accomodate the mirror beam splitter therebetween, for directing a portion of the light from the output phosphor passing through the collimating lens, to the second film camera disposed transverse from the optical axis of the system. This spacing between the two lens elements contributes to vignetting (loss of brightness in the peripheral areas of the image with respect to central brightness) of the image passed to the television pickup tube.
Other structure has also been designed to provide for direct photography of the image at the output phosphor of the image tube. According to this other structure, the second film camera is aligned with the optical axis of the image tube, and the television pickup tube is disposed transverse to that axis. A diverter is provided in the optical axis, positioned between a set of lenses for transmitting and focusing the output phosphor image. The diverter deflects light energy from the output phosphor to the television camera in synchronism with the field scan rate of the television pickup tube. During the remainder of time, the diverter allows the unimpeded passage of the output phosphor energy to the film camera. Such a system is described in the copending application Ser. No. 537,776, filed Dec. 31, 1974 by Fred H. Meyer and assigned to the assignee of this application.
Both these systems for deploying the second film camera require the camera to be mounted in a fixed spatial relation to the image tube, lens assembly, television pickup tube and, with respect to the system of application number 537,776, the diverter.
The sensitivity and resolution of the prior systems has been improved somewhat by the use of highly sensitive image isocon television pickup tubes. The image isocon includes an input face (photocathode) for emitting electrons in accordance with the brightness distribution of the viewed visual image, and a glass target against which the electrons from the photocathode are directed. A conductive mesh is interposed between the photocathode and the target, being displaced approximately 25 microns from the target. The charge distribution on the target varies corresponding to the viewed image, i.e., to the pattern of electrons directed onto the target by the photocathode. An electron gun produces a reading beam current of electrons which is scanned across the target line by line. Part of the beam current is reflected back away from the target, and the reflected beam current varies with respect to the potential distribution on the target resultant from the charge pattern. By sensing the instantaneous charge flow rate of electrons reflected from the target, an electrical signal is derived representing in analog fashion the instantaneous relative brightness of each portion of the viewed image being scanned.
The inherent resolution of the image isocon television pickup tube is enhanced by a phenomenon known as "redistribution." When electrons from the photocathode strike the target, they cause the emission from the target of between three and four times as many secondary electrons. Normally these secondary electrons migrate to the mesh, which is normally maintained at a potential about 3 volts more positive than the target. This results in an accumulation of net positive charge in the region of the target during a continuous influx of electrons from the photocathode to the target. When the net charge in the target region becomes generally positive, the secondary electrons, rather than moving toward the mesh, accumulate and "redistribute" themselves at nearby locations on the target. This redistribution is such that the charge distribution areas on the target representing dark areas become darker, and those areas representing lighter areas become lighter. This generally enhances the outlines of the light and dark portions of the image, as represented by the target potential pattern and by the video signal which is produced by the reading beam. This feature is particularly advantageous in X-ray systems, because of inherent low detail and low contrast of the visible image at the image tube output phosphor.
The image isocon, however, loses a significant portion of its advantages when used in a pulsed X-ray system, wherein the pickup tube scan rate is equal to the pulse rate, i.e., about 1-6 cycles per second. The image isocon exhibits degraded redistribution in the pulsed mode of operation. It is not fully understood why the image isocon exhibits pronounced redistribution only in substantially continuous operation, but this in nonetheless a fact, and inhibits its utility in pulsed X-ray applications.
Resolution was also reduced by the lack of resolving power and limitations on image quality of the CRT television display apparatus. The CRT displays used in the past have had generally rounded convex faces, making them difficult to optically couple with lenses of cameras and other optical processing apparatus, and contributing to optical distortion of recorded television images. Moreover, those phosphors commonly used in the CRT (e.g., "P4" phosphor, emitting white light), have not been ideal for use in exposing most films, including the newer type dry process films, which are most sensitive to blue light. The use of phosphors fluorescing in other than blue light has required the use of higher beam currents to obtain needed brightness. As is known, resolution of a CRT decreases as beam current increases.
At normal beam currents, the focal spot size of the electron beam of those prior CRT's ranged between 0.010 and 0.020 inches, which is too large for resolution required for producing relatively high resolution images.
The prior system had very substantial mass and bulk, due to the need for mounting the second film camera on the unit in an integral fashion with the other system elements to maintain its disposition for making the direct high resolution photos of the output phosphor image. This requirement increased the complexity, size and cost of equipment for supporting and maneuvering the system about the patient's body.
The prior system lacked flexibility of use, in that remote viewing of real-time high resolution images was not possible. Rather, the film had to be removed from the second film camera and processed before viewing, a time-consuming and annoying process which often extended the time the patient had to remain in the examination room.
The size of the prior system and of the mechanism for moving it sometimes caused patients undue anxiety.
In the past, the size and mass of the imaging system, and the need for moving it efficiently about the patient, generated much effort in providing mechanisms for support and easy motion. Such support apparatus included tall tower structures and complex counterweighting systems. As systems grew heavier, the problems of support and motion became more acute, and the mechanisms substantially increased the amount of space required in the examination room. Great care had to be used to mount the apparatus for safe support of the heavy imaging systems.
It is a general purpose of this invention to provide a high resolution, low noise compact television imaging system for producing high quality television images from visible light images from an ordinary X-ray image intensifier tube.